33 research outputs found
Non-Markovian Memory Strength Bounds Quantum Process Recoverability
Generic non-Markovian quantum processes have infinitely long memory, implying
an exact description that grows exponentially in complexity with observation
time. Here, we present a finite memory ansatz that approximates (or recovers)
the true process with errors bounded by the strength of the non-Markovian
memory. The introduced memory strength is an operational quantity and depends
on the way the process is probed. Remarkably, the recovery error is bounded by
the smallest memory strength over all possible probing methods. This allows for
an unambiguous and efficient description of non-Markovian phenomena, enabling
compression and recovery techniques pivotal to near-term technologies. We
highlight the implications of our results by analyzing an exactly solvable
model to show that memory truncation is possible even in a highly non-Markovian
regime.Comment: 8 pages, 7 pages of appendices, 5 figures. Close to the published
versio
Hidden Quantum Memory: Is Memory There When Somebody Looks?
In classical physics, memoryless processes and Markovian statistics are one
and the same. This is not true for quantum processes, first and foremost due to
the fact that quantum measurements are invasive. Independently of measurement
invasiveness, here we derive a novel distinction between classical and quantum
processes, namely the possibility of hidden quantum memory. While Markovian
statistics of classical processes can always be reproduced by a memoryless
dynamics, our main result establishes that this is not the case in quantum
mechanics: We first provide an example of quantum non-Markovianity that depends
on whether or not a previous measurement is performed -- a phenomenon that is
impossible for memoryless processes; we then strengthen this result by
demonstrating statistics that are Markovian independent of how they are probed,
but are are nonetheless still incompatible with memoryless quantum dynamics.
Thus, we establish the existence of Markovian statistics that fundamentally
require quantum memory for their creation.Comment: 4.5 + 8.5 pages, 3 figure
Characterising the Hierarchy of Multi-time Quantum Processes with Classical Memory
Memory is the fundamental form of temporal complexity: when present but
uncontrollable, it manifests as non-Markovian noise; conversely, if
controllable, memory can be a powerful resource for information processing.
Memory effects arise from/are transmitted via interactions between a system and
its environment; as such, they can be either classical or quantum in nature.
From a practical standpoint, quantum processes with classical memory promise
near-term applicability: they are more powerful than their memoryless
counterpart, yet at the same time can be controlled over significant timeframes
without being spoiled by decoherence. However, despite practical and
foundational value, apart from simple two-time scenarios, the distinction
between quantum and classical memory remains unexplored. We first analyse
various physically-motivated candidates regarding a suitable definition for
classical memory that lead to remarkably distinct phenomena in the multi-time
setting. Subsequently, we systematically characterise the hierarchy of
multi-time memory effects in quantum mechanics, many levels of which collapse
in the two-time setting, thereby making our results genuinely multi-time
phenomena.Comment: 11+5 pages, 4 figures, 57 reference
Landauer vs. Nernst: What is the True Cost of Cooling a Quantum System?
Thermodynamics connects our knowledge of the world to our capability to
manipulate and thus to control it. This crucial role of control is exemplified
by the third law of thermodynamics, Nernst's unattainability principle, stating
that infinite resources are required to cool a system to absolute zero
temperature. But what are these resources and how should they be utilised? And
how does this relate to Landauer's principle that famously connects information
and thermodynamics? We answer these questions by providing a framework for
identifying the resources that enable the creation of pure quantum states. We
show that perfect cooling is possible with Landauer energy cost given infinite
time or control complexity. However, such optimal protocols require complex
unitaries generated by an external work source. Restricting to unitaries that
can be run solely via a heat engine, we derive a novel Carnot-Landauer limit,
along with protocols for its saturation. This generalises Landauer's principle
to a fully thermodynamic setting, leading to a unification with the third law
and emphasising the importance of control in quantum thermodynamics.Comment: 15 pages, 4 figures, 46 pages of appendice
Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis
Large-scale atmospheric removal of greenhouse gases (GHGs) including methane, nitrous oxide and ozone-depleting halocarbons could reduce global warming more quickly than atmospheric removal of CO2. Photocatalysis of methane oxidizes it to CO2, effectively reducing its global warming potential (GWP) by at least 90%. Nitrous oxide can be reduced to nitrogen and oxygen by photocatalysis; meanwhile halocarbons can be mineralized by red-ox photocatalytic reactions to acid halides and CO2. Photocatalysis avoids the need for capture and sequestration of these atmospheric components. Here review an unusual hybrid device combining photocatalysis with carbon-free electricity with no-intermittency based on the solar updraft chimney. Then we review experimental evidence regarding photocatalytic transformations of non-CO2 GHGs. We propose to combine TiO2-photocatalysis with solar chimney power plants (SCPPs) to cleanse the atmosphere of non-CO2 GHGs. Worldwide installation of 50,000 SCPPs, each of capacity 200 MW, would generate a cumulative 34 PWh of renewable electricity by 2050, taking into account construction time. These SCPPs equipped with photocatalyst would process 1 atmospheric volume each 14â16 years, reducing or stopping the atmospheric growth rate of the non-CO2 GHGs and progressively reducing their atmospheric concentrations. Removal of methane, as compared to other GHGs, has enhanced efficacy in reducing radiative forcing because it liberates more °OH radicals to accelerate the cleaning of the troposphere. The overall reduction in non-CO2 GHG concentration would help to limit global temperature rise. By physically linking greenhouse gas removal to renewable electricity generation, the hybrid concept would avoid the moral hazard associated with most other climate engineering proposals